refactor(router/atc): iterate and append items properly when splitting routes

[chronolaw]

Summary

We iterate items from 1 to routes_and_services_count, but in the loop
routes_and_services_count may be changed, it is not our expected.

The variable routes_and_services_count should not be changed,
so we could use another variable appending_index, which has the same effect.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Issue reference

Fix #[issue number]

[bungle]

it is not a bug, for i=x, variable is evaluated only once

[chronolaw]

Got, thanks.